It is well known to have male and female coupling members removably locked together by a peripheral groove in one member and a pair of pivoted cam members on the other member adapted to engage the peripheral groove. The desirability of holding the cam members in the locking position is recognized by the prior art. Reference may be had for example to U.S. Pat. No. 3,976,313 in which detents are employed to hold levers on such cam members in closed position against inadvertent rotation. In U.S. Pat. No. 3,124,374 a similar structure is disclosed in which rotation of a lever on a cam member is prevented by a pin having an axis parallel to the axis of the coupling and mounted in a spring biased plunger with an axis perpendicular to the axis of the pin in such a manner that depressing the plunger clears the pin away from the cam member lever to permit its rotation. The use of a detent as in U.S. Pat. No. 3,976,313 is risky since a relatively small force can pop the cam member free of the detent and thus permit the unlocking of the coupling. The structure of U.S. Pat. No. 3,124,374 is relatively expensive and is subject to malfunctioning incident to being invaded by dirt. These disadvantages have been overcome by providing a locking device in which all the parts are integral with the coupling except for a removable wire pin which holds the cam lever securely since the only way the cam lever can come loose is to shear the pin. The device of the invention is simple, inexpensive and effective. The only loose part is the pin which is very cheap and can be replaced at little cost if lost.